Static source NAT fails for outgoing through different gateways with SDWAN

[BensonLEI]

Hi, SDWAN guys,

We are using Forti400e with FortiOS v6.4.4;  the Fortigate 400E HA-pair has three ISP links ( ISP01, ISP02 and ISP03) and it uses the SD-WAN rules "Maximize Bandwidth (SLA)", and the SD-WAN configuration is as below:

Forti400E_2 # get sys sdwan                      status : enable                      load-balance-mode : source-dest-ip-based                      duplication-max-num : 3                      neighbor-hold-down : disable                      neighbor-hold-down-time: 0                      neighbor-hold-boot-time: 0                      fail-detect : disable

My finding is similar to the following issue ( but we are using SD-WAN -- modern term :(

 Static NAT fails for outgoing connections through gateway with ISP Redundancy in Load Sharing mode (...

The Fortigate tools used:

1. sniffier

Forti400E_2 # diagnose sniffer packet any 'host 139.162.19.237' 4 interfaces=[any] filters=[host 139.162.19.237] ... port2 out 212.00.00.5.51646 -> 139.162.19.237.443: syn 3266995216 port4 out 212.00.00.5.51627 -> 139.162.19.237.443: syn 134275028 port15 out 212.00.00.5.51645 -> 139.162.19.237.443: syn 2660092068 

port15 in 139.162.19.237.443 -> 212.00.00.5.51628: syn 453013466 ack 2660092069 

...

2. session list ( nothing helpful )

    Forti400E_2 # diag debug disable

    Forti400E_2 # diag debug flow filter addr 139.162.19.237

    Forti400E_2 # diag debug flow trace start 500

    Forti400E_2 # diag debug enable

3. route table ( all are correct )

by "diag ip rtcache list"

by " get router info routing-table all"

..

Problem ( it should be the ISP line problem, but not the Fortigate SD-WAN issue :(

When I tested different ISP IP( for outgoing NAT ) by "IP pools" from "Policy & Object"...some ISP lines do not let traffic out; 

My questions:

1.  I would like to search for the knowledge in depth of the SDWAN algorithm, any recommendation ?

2.  Any fortigate built-in tool for inspecting this issue ?

Any advice and recommendation ?

Many thanks in advance